Oceanographic and environmental concerns have increased the need for chemical sensing devices for the measurement of small-scale chemical features in aquatic environments. Conventional methods usually resort to two approaches; namely the collecting and analyzing the discrete samples in the laboratory and the use of sensors for in situ analysis. Disadvantages of both approaches for measuring nutrient distributions, species of geochemical interest and toxical chemicals are that they usually require cumbersome, individualized sampling techniques. Often the required sample handling and processing are time consuming, labor-intensive and may be subject to contamination and storage problems because some samples are often analyzed days to weeks after collection. In addition, such a delay does not lend itself well for mapping distributions of chemical constituents in dynamic aquatic environments. Presently available in situ sensors (mostly based on electrochemical measurements) are limited by several factors. Their response times may be slow, memory effects may persist, electrical interferences may compromise the results and some sensors are available only for a limited number of constituents. Optically based in situ sensors (optrodes) have relied upon chemical indicators (ligands) immobilized on the end of an optical fiber. When an analyte ion or molecule complexes with the ligand, fluorescence is induced or quenched. Such sensors have been used for measuring pH as reported by J. I. Petersen, et al in their article on page 864 in Analytical Chemistry 52 (1980) and D. M. Jordan, et al in their article on page 437 in Analytical Chemistry 59 (1987). Such sensors also have been found to provide indications in dissolved gases as reported by G. G. Vurek, et al on page 499 of Anals Biomed. Enqr. 11 (1983) and J. I. Peterson, et al's article on page 62 of Analytical Chemistry 56. Metal ions also have been sensed based on this type of a sensor as reported by S. Zhujun, et al in their article appearing on page 251 of Anal. Chim. Acta 171 (1985) and the L. A. Saari, et al article on page 667 of Analytical Chemistry 55 (1983). Such sensors typically involve the immobilization of a fluorogenic indicator on or near the end of a fiber optic cable. Excitation light and stimulation of the fluorophore and the resultant emission signal are both transmitted through the fiber optic cable. A problem with using immobilized ligands for real time sensing is that they are subject to photodegradation, leaching from the immobilization substrate, and there are difficulties with the immobilization chemistry. Most importantly with few exceptions, reversibility has not been demonstrated for immobilized ligand sensors, therefore they cannot be used for real time sensing.
Thus a continuing need exists in the state of the art for a sensor system that avoids the problems associated with immobilized ligand systems by forcing the ligand in a solution through a membrane and measuring the fluorescence or quenching that occurs at the membrane surface where the ligand interacts with the analyte solution.